1. Field of the Invention
The present invention relates to a development apparatus used for an image forming apparatus such as a copying machine and a printer utilizing an electrophotographic system.
2. Description of the Related Art
There has been a magnetic brush development method to perform development utilizing two-component developer as a development apparatus utilized for a conventional image forming apparatus such as a copying machine, a printer and a facsimile machine.
In the magnetic brush development method, only toner particles are consumed for image forming and carrier particles are to be worn as circulating in a development apparatus. Therefore, such a development apparatus is provided with a mechanism to replenish replenisher obtained by mixing toner and carrier at a predetermined ratio as discharging a part of developer according to operation of the development apparatus. Accordingly, it is possible to suppress the wear degree of the carrier staying in the development apparatus within a certain range while replenishing toner consumed according to image forming.
With such a mechanism of replenishing and discharging developer (i.e., an auto carrier refreshing mechanism (ACR)), replenishment is normally performed as follows. That is, a mixture ratio between the toner and carrier is detected by a toner concentration sensor which is disposed in the development apparatus and the replenishment amount is adjusted so that the detection result becomes a desired mixing ratio. Since a toner mixing ratio of replenisher is larger than a toner mixing ratio of developer in the development apparatus in normal cases, the toner mixing ratio in the development apparatus is increased with replenishment.
Meanwhile, as disclosed in Japanese Patent Laid-open No. 59-100471, in view of mechanism simplification, it is popular to adopt a method for discharging to simply form a hole (i.e., a discharge hole) at a predetermined position in a development apparatus and to make developer of which bulk is increased with replenishment overflow therefrom. When bulk density of the developer is constant, the present mechanism has a function to define the upper limit of the developer amount in the development apparatus. Meanwhile, provided that the developer is sufficiently agitated, the toner mixing ratio is not varied. Accordingly, behavior thereof is easily assumed.
In Japanese Patent Laid-open No. 10-186855, a discharge port is formed at a position adjacent to a replenishing port for replenishing replenisher at the upstream side in a developer conveyance direction. With this structure, replenisher is prevented from being discharged through the discharge port in a state of having an unstable toner mixing ratio without being sufficiently agitated.
It is preferable that the total developer amount staying in the development apparatus as disclosed in Japanese Patent Laid-open No. 59-100471 and Japanese Patent Laid-open No. 10-186855 is constant or at least in a predetermined range. An excessive amount of developer causes overflowing from the development apparatus according to variation of developer flowability. On the other hand, when the developer amount is too low, a sufficient amount of the developer is not supplied to an entire imaging area of a development sleeve to cause missing of an output image in part. Accordingly, it is ideal that discharging is not performed at all until the developer amount reaches a certain level and only an amount of developer exceeding a threshold value is promptly discharged when the developer amount exceeds the threshold value.
However, in actual fact, there is a case that a developer discharge amount cannot be controlled appropriately with variation of bulk density and flowability of developer due to developer wear and variation of ambient temperature and humidity. Further, when air pressure in a development apparatus becomes negative against outside pressure, airflow to draw air at the vicinity of a discharge port to the inside of the development apparatus occurs and developer becomes difficult to be discharged.
Here, positional relation between the discharge port and the replenishing port of toner in the development apparatus is arranged so that discharging is performed while replenished toner is sufficiently agitated. The replenisher replenished from the replenishing port is conveyed to a developer circulation passage by a conveying unit disposed outside the developer circulation passage before being merged to the developer circulation passage in the development apparatus. That is, there is a conveying unit disposed to convey only replenisher which in not mixed yet to the developer circulating in the development apparatus.
In such a system, when the replenishing port is hermetically-closed, air at the vicinity of the replenishing port is sucked by the above-mentioned conveying unit and negative pressure is generated at the vicinity of a merging point with the developer circulation passage. On the contrary, when the replenishing port is opened, the above phenomenon does not occur. In this manner, air pressure in the development apparatus depends on airtightness at the replenishing port.
When the pressure at the vicinity of the merging point with the developer circulation passage becomes negative, developer existing at the upstream side thereof is drawn toward the downstream side. Normally, the discharge port is located at the upstream side of the replenishing port to prevent replenisher from being discharged without being agitated. Accordingly, in a case with negative pressure, the developer existing at the vicinity of the discharge port is drawn and developer discharging is disturbed.
In this manner, easiness of developer discharging is varied according to airtightness at the replenishing port. Therefore, the airtightness at the replenishing port is required to be kept constant to stabilize the developer amount in the development apparatus. Normally, the replenishing port is appropriately closed to prevent leakage of replenisher from a gap. However, on the other hand, it is difficult to maintain complete airtightness with a detachable development apparatus. The degree of airtightness at the replenishing port is varied according to slight variation of an attaching position of the development apparatus. The above causes variation of easiness of developer discharging and variation of the developer amount staying in the development apparatus. Then, poor imaging is caused owing to developer overflowing and developer supplying shortage.